Paul Booth at the Democratic National Convention in 2016, holding a copy of the party platform he helped draft. (J. Scott Applewhite/AP)

Paul Booth, a progressive activist who organized one of the first major rallies against the Vietnam War — a 15,000-student march on the White House in 1965 — and who later oversaw efforts to boost wages and preserve Social Security benefits as a top strategist for one of the nation’s largest unions, died Jan. 17 at a hospital in Washington. He was 74.

The cause was complications from chronic lymphocytic leukemia, said his wife, activist Heather Booth. On the day he died, Mr. Booth was working on an article for the American Prospect and had encouraged his wife to attend a Capitol Hill demonstration, where she was arrested while protesting on behalf of “dreamers” protected by the Deferred Action for Childhood Arrivals program.

The lanky son of a left-leaning economist and social worker, Mr. Booth engrossed himself in organizing as a student at Swarthmore College in Pennsylvania, where in the early 1960s he founded a chapter of Students for a Democratic Society — a fractious, sometimes anarchic organization whose calls for peace, social justice and political reform came to define the movement known as the New Left.

“We’re really not just a peace group,” he told the New York Times on April 17, 1965, the day he led the SDS-organized war protest in Washington. “We are working on domestic problems — civil rights, poverty, university reform. We feel passionately and angrily about things in America, and we feel that a war in Asia will destroy what we’re trying to do here.”

Mr. Booth was described by Alan Haber, first president of SDS, as a rare “cheerful spirit” in the organization, singing and telling stories to maintain morale during the contentious drafting process that resulted in the organization’s 1962 manifesto, the Port Huron Statement, under student leaderTom Hayden.

Mr. Booth and his wife, Heather Booth, at an AFSCME event in 2012. (Khalid Naji-Allah/AFSCME)

Rising to the position of national secretary, the group’s de facto leader, he worked with civil rights and women’s groups to organize events such as the antiwar march on the White House and an earlier rally at the New York headquarters of Chase Manhattan Bank, which the coalition labeled a “partner in apartheid” for giving loans to the South African government.

Still, Mr. Booth was viewed as insufficiently radical by many of the organization’s younger members. When he espoused a policy of “build, not burn,” recommending that young Americans perform volunteer work or humanitarian service in lieu of military service or draft-card burning, he was censured by some SDS leaders.

Sociologist Todd Gitlin, a fellow SDS activist who helped organize the bank and antiwar rallies, described Mr. Booth’s politics as akin to what writer and socialist leader Michael Harrington called “the left wing of the possible”: “Don’t go out on a limb, don’t break your contact with ordinary people and mainline institutions.”

A protege of community organizer Saul Alinsky, Mr. Booth left SDS to become a labor organizer in 1966. He worked on environmental efforts in Chicago before joining the American Federation of State, County and Municipal Employees, the country’s largest public-services employees union.

He went on to serve as the chief assistant to union president Gerald W. McEntee and as an executive assistant to his successor, Lee Saunders. His titles belied the full range of his work, which included behind-the-scenes efforts to support Democratic politicians and maintain or expand social benefits for working-class families.

In addition to leading campaigns that opposed cuts to Medicare and the privatization of Social Security, Mr. Booth was credited with organizing a coalition in Baltimore that successfully pressed for the country’s first living-wage law. Passed in 1994, the legislation raised the base pay for city contract workers in Baltimore above the federal minimum wage and has since been mimicked in cities across the country.

The law also formed the seeds of the recent campaign for a $15 minimum wage — an issue that made it onto the Democratic Party’s official platform at the 2016 national convention. Mr. Booth, selected by presidential nominee Hillary Clinton, was a member of the committee that wrote the platform.

Paul Robert Booth was born in Washington on June 7, 1943. His father was a Labor Department economist who later worked for the International Labor Organization in Geneva; when he suffered a heart ailment, the family returned to the United States and became dependent on his mother’s income as a social worker.

Mr. Booth graduated from Woodrow Wilson High School in the District and, in 1964, received a bachelor’s degree in political science from Swarthmore.

He met Heather Tobis two years later at a University of Chicago sit-in protesting the Selective Service, and after three days on the floor of the school’s administration building asked her to marry him. She later formed the Midwest Academy, a Chicago-based training center for social-justice organizers, where Mr. Booth was a board member.

In addition to his wife of 50 years, survivors include two sons, Gene Booth of Chicago and Dan Booth of Concord, Mass., named for the socialist leaders Eugene V. Debs and Daniel De Leon, respectively; a brother; and five grandchildren.

Mr. Booth was research director for the United Packinghouse Workers of America before joining AFSCME in 1974. As the international union representative for Illinois, he secured union contracts for state workers and city employees in Chicago — “further speeding the demise of the patronage system,” the Chicago Tribune reported in 1988.

He retired in 2017 but remained engaged in national politics, even when he was hospitalized last week. On a CaringBridge site for Mr. Booth created shortly before his death, his wife noted that alongside notes or calls from well-wishers, “Paul particularly welcomes any news about more Republican retirements.”

What he really wanted, she continued, was for friends and strangers “to organize and build the resistance.”